Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description still adds value by stating "Changes nothing" and the auth-ish precondition "Needs the user's account," plus listing the returned fields despite no output schema.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three short sentences, front-loaded with the trigger condition, then output contents, then the side-effect/auth note. No filler and nothing redundant with the title.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a parameterless read with no output schema, the description supplies exactly the missing pieces: when to call it, what it returns (credit, currency, minimum top-up), and that it has no side effects. An agent has everything needed to call it correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Zero parameters, so the baseline is 4. The description adds the only meaningful input context — that the user's account is needed — which is implicit scoping rather than a formal parameter. Nothing further is required.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb+resource (get the prepaid credit balance) and enumerates what comes back: credit, account currency, minimum top-up. This is unmistakable against siblings like create_topup_link or get_price_quote, which serve different purposes.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives two concrete triggering conditions: the user asking about prepaid credit, and checking coverage before sending a letter. That is clear context, but there is no explicit when-not guidance or named alternative for the related money question (top-up vs. price quote).

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.